This invention relates to a method and apparatus for the deposition of thin, hard, wear-resistant diamond-like carbon and silicon-doped diamond-like carbon coatings using gridless Hall-Current ion sources, and the use of the process to produce protective coatings for data storage and other applications.
There are numerous examples in the prior art of the application of gridded ion sources to the direct deposition of diamond-like carbon coatings (DLC) from hydrocarbon gases, mixtures of hydrocarbon gases and mixtures of inert gases with hydrocarbon gases within the ion source. Two recent articles help to illustrate state-of-the-art application of gridded ion sources to the deposition of DLC coatings. M. Weiler et al., Applied Physics Letters, Vol. 64, pages 2792-2799 (1994), and Sattle et al., Journal of Applied Physics, Vol. 82, pages 4566-4575 (1997) describe application of a gridded ion source to the direct deposition of DLC using acetylene gas. Their work shows that DLC coatings with high sp3 fraction and with hardness greater than 10 GPa can be deposited at moderate deposition rates up to about 10 xc3x85 per second provided that (1) a high degree of ionization of the feed gas is obtained, (2) a high C/H ratio of ion species is formed and (3) the mean energy of the beam provides about 100 eV per C atom deposited from the ion beam. As such, gridded ion sources typically operate with ion energies near 100 to 250 eV in order to form hard (i.e.  greater than 10 GPa) DLC coatings.
It is also known that one may produce DLC coatings which incorporate other dopant elements, e.g. silicon. Such coatings are commonly referred to as silicon-doped DLC or Si-DLC. For example, Brown et al., pending patent application U.S. Ser. No. 08/707,188 filed Sep. 3, 1996, disclose a Si-DLC coating for magnetic transducers and magnetic recording media which is deposited by from silicon-containing and carbon-containing precursor gases by a direct ion beam deposition method. These Si-DLC coatings are characterized by the following features: Nanoindentation hardness in the range of about 12 GPa to 19 GPa, compressive stress in the range of about 0.4 GPa to 1.8 GPa, Raman spectral G-peak position in the range of about 1463 cmxe2x88x921 to about 1530 cmxe2x88x921, a silicon concentration in the range of about 1 atomic % to about 30 atomic % and hydrogen concentration of about 25 atomic % to about 47 atomic %. In their examples, a gridded ion source is disclosed as the apparatus to deposit Si-DLC coatings.
However, it is well known that gridded ion sources are limited in DLC and Si-DLC deposition applications by the function of the electrostatic grid optics which are necessary to form near mono-energetic ion beams. In production, such grids limit the beam current density and, thereby, the deposition rates. Also the grids become coated substantially with deposits which eventually disrupt production, limit maintenance cycles, and cause extensive maintenance problems when removing such deposits from the grid optics. Gridded ion sources operate at low vacuum pressures, typically below 5xc3x9710xe2x88x924 Torr, and typically have ion beam DLC deposition rates of less than 10 xc3x85/sec for most all variety of gaseous hydrocarbon chemistries.
In order to overcome the limitations with gridded ion sources, prior art attempts have been made to use gridless Hall-Current ion sources for the deposition of DLC coatings. In these DC or pulsed-DC devices, ions are accelerated from a region of ion production through an electric field, E, established within the bulk of the discharge near the anode of the apparatus. The electric field is brought about by a static magnetic field, B, imposed on the discharge in the vicinity of an anode wherein the electron drift motion from cathode to anode is impeded by the magnetic field. Electrons emitted from the cathode ionize feed gases as they drift toward the anode through the magnetic field via collisional and anomalous diffusion. The restricted mobility of electrons across the magnetic field lines forms a space-charge near the anode and an electric field that is substantially orthogonal to the imposed magnetic field. Ions generated within the anode discharge region are accelerated away from the anode. Since the anode discharge and ion acceleration regions do not exclude electrons, ion beam current densities are not restricted by space-charge limitations that are inherent in electrostatic acceleration optics. A fraction of the electrons emitted from the cathode and those released within the discharge from ionization also serve to electrically neutralize the ion beam as it propagates away from the anode""s ion acceleration region. At pressures above 10xe2x88x924 Torr, ionization away from the anode discharge region and charge-exchange processes within the ion beam can form a diffusive background discharge making the output characteristics of the source appear as both an electrically neutralized ion beam and a quasi-neutral diffusive plasma. Unlike the near mono-energetic ion energy distribution of a gridded ion source, this gridless ion source has a broad energy spectrum and is capable of very high ion current densities. The combined output of both a self-neutralized ion beam and diffusive plasma is sometimes referred to as a xe2x80x9cplasma beamxe2x80x9d.
Another characteristic feature of this type of ion source is an Exc3x97B drift current motion of electrons in the anode discharge region. Electrons, which spiral about the lines of the magnetic field, experience an Exc3x97B or Hall-effect force and collectively drift in a direction perpendicular to both magnetic and electric fields. This is referred to as a Hall-effect drift current. In order to avoid Hall potentials which may form along this electron drift path, these ion sources have anode discharge regions or channels that allow Hall-effect current to drift along a continuous and closed path. The prior art refers to these types of ion sources by many names: xe2x80x9cMagneto-plasma-dynamic Arc Thrustersxe2x80x9d, xe2x80x9cHall Acceleratorsxe2x80x9d, xe2x80x9cClosed-Drift Thrustersxe2x80x9d, and xe2x80x9cHall-Current Ion Sourcesxe2x80x9d. For the purpose of this disclosure, these devices are referred to, in general, as xe2x80x9cHall-Current ion sourcesxe2x80x9d. Yet another version of this technology is the xe2x80x9cEnd-Hallxe2x80x9d ion source described by Kaufman et al, U.S. Pat. No. 4,862,032, issued Aug. 29, 1989.
The following references illustrate the prior art with regard to the application of Hall-Current ion sources to the deposition of DLC coatings.
Okada et al., Japanese Journal of Applied Physics, Vol. 31, pages 1845-1854 (1992), describe a high energy Hall-Current ion source used for ion implantation and deposition of hard, abrasion resistant DLC coatings.
Fedoseev, et al. Diamond and Related Materials, Vol. 4, pages 314-317 (1995), describe a Hall-Current ion source used in deposition of relatively transparent DLC coatings.
Baldwin, et al., U.S. Pat. No. 5,616,179, issued Apr. 1, 1997, describe a process for the deposition of diamond-like, electrically conductive, and electron-emissive carbon-based films and coatings using the End-Hall ion source of the Kaufman et al. ""032 patent.
Knapp et al., U.S. Pat. No. 5,508,368, issued Apr. 16, 1996 and Petrmichl et al., U.S. Pat. No. 5,618,619, issued Apr. 8, 1997 describe a process in which an End-Hall ion source or Hall-Current ion source is used to deposit highly transparent coatings comprised of C, Si, O and H, with Nanoindentation hardness of about 2 to 5 GPa, and having abrasion resistance comparable to glass.
It is desirable to deposit DLC and Si-DLC coatings in a production setting on a wide variety of substrates for many applications by direct ion beam deposition with a gridless Hall-Current ion source. More specifically, it is desirable to deposit DLC and Si-DLC coatings from a gridless Hall-Current ion source wherein such coatings have Nanoindentation hardness values greater than 10 GPa and wherein the deposition rates of such coatings are greater that 10 xc3x85 per second in a production setting. As an example, it is desirable to produce ultra-thin, e.g. less than 100 xc3x85 thick, preferably about 20 xc3x85 to 50 xc3x85 thick DLC and Si-DLC overcoats with hardness greater than 10 GPa on magnetic transducers and magnetic media used in data storage applications to provide the required tribological performance of the head-disk interface while maintaining very low magnetic spacing. In high volume production, it is desired that the overcoats on magnetic disks be deposited at rates of about 5 to 30 xc3x85 per second in order to meet production throughput requirements.
In addition to optimization of the properties of the DLC and Si-DLC, it equally important to be able to apply these coatings in a production environment. Thus, it is necessary that the Hall-Current ion source have the following additional characteristics and capabilities when depositing DLC and Si-DLC coatings:
1) operate at relatively high power and discharge current levels, be relatively insensitive to the nonconductive hydrocarbon-coatings that form within the anode discharge region of the ion source,
2) have a beam output that is reliable, consistent and symmetric with respect to the ion source geometry,
3) be free of potentially damaging arcing and other undesirable electrical transient events,
4) be easy to ignite and apply to periodic on-off operation, and
5) be easily scalable to applications involving large surface areas.
Prior art Hall-Current ion sources all suffer from limitations which inhibit their use for the deposition of DLC and Si-DLC coatings in a production environment. None of the prior art references teach how to deposit DLC and Si-DLC coatings with hardness greater than 10 GPa and at deposition rates substantially greater than 10 xc3x85 per second. Furthermore, prior art references do not teach how to overcome problems associated with high-rate deposition of nonconductive DLC and Si-DLC coatings in a production environment.
Okada et al., describe the use of a relatively high beam energy (about 200 to 1000 eV) Hall-Current ion source in the deposition of DLC films from methane, but do not discuss the application of the apparatus to Si-DLC coatings. Their bulky and sophisticated device uses many electromagnets to form a magnetic field within its extended acceleration channel. There is no direct, active cooling of the anode assembly and the apparatus is operated at low gas flow rate of a few sccm, and a low anode-to-cathode current near 1 Amp. Using an ion beam energy on the order of 500 eV, the peak Vickers hardness values of coatings deposited from methane at 10 cm downstream of the face of the ion source were high, about 4600 kg/mm2 Vickers hardness with a 10 gram load. However, the deposition rate was on the order of only 1 to 2 xc3x85 per second, which is far lower than desired. There is no teaching on how to operate or configure this source to produce DLC or Si-DLC at high deposition rates. Moreover, this type of Hall-Current ion source does not have an independent cathode that generates a self-sustained and continuous emission of electrons. This makes periodic on-off operation difficult in production. As such there are no disclosed reports or embodiments within this extended-channel Hall-Current ion source that suggest that it can deposit high hardness DLC coatings at high rates or that it can address common problems encountered in direct deposition of coatings at high rates.
Fedoseev et al. used a mixture of hydrogen and hydrocarbon feed gases in a Hall Accelerator ion source with a water-cooled anode to produced DLC coatings. Their intent was to produce optically transparent DLC coatings with low levels of light absorption. DLC coatings produced from a feed gas of hydrogen plus methane, ethane and acetylene were deposited on substrates positioned 18 cm from the face of the ion source. Deposition rates ranged from about 2 to 30 xc3x85 per second. However, Vickers hardness of the DLC coatings was very low, in the range of 400 to 1,000 kg/mm2 (4 to 10 GPa) measured with a very light load of 5 grams. Further, it is known that the DLC coating hardness is overestimated by the use of such low indenter loads. As such, there is no teaching to suggest how to achieve very high hardness DLC coatings by means of Fedoseev""s Hall Accelerator ion source.
Baldwin et al. in the ""179 patent describe a process for depositing amorphous or nanophase DLC coatings using an End-Hall ion source. In one embodiment of this process, various hydrocarbon gases in combination with argon are used to deposit DLC coatings at rates as high as about 33 xc3x85 per second with hardness of 8 GPa. In their preferred embodiment, Baldwin et al. teach that DLC films are produced by the End-Hall ion source from methane/argon mixtures with an ion energy of about 90 to 100 eV per C atom deposited and a beam current density of about 0.2 A/cm2. No other teachings are presented that suggest how to modify the process conditions or the configuration of the End-Hall ion source so as to enable the deposition of very hard DLC or Si-DLC coatings at high deposition rates.
Knapp et al. and Petrmichl et al., in the ""368 and ""619 patents, respectively, describe an ion beam deposition method to manufacture a substance with improved abrasion resistance and improved lifetime. In the detailed description of these inventions, examples and preferred embodiments, an End-Hall ion source is operated with hydrocarbon gases including methane and cyclohexane to deposit DLC. These patents primarily focused on the deposition of highly extensible, abrasion resistant coatings with preferred hardness in the range of about 2 to 5 GPa and low compressive stress. Knapp et al. also allude to the application of other types of Hall-Current ion sources (i.e. Hall Accelerator ion sources) for this method, but do not discuss or teach how such alternative gridless ion sources should be designed, configured or operated in order to carry out the disclosed deposition method. As such, there are no teachings which suggest how to apply Hall-Current ion sources to form high hardness DLC or Si-DLC coatings at high rates in production.
Yet other operational shortcomings of prior art Hall-Current ion sources are described by Mahoney et al. in co-pending patent application U.S. Ser. No. 08/901,036 filed in Jul. 25, 1997, which description is incorporated herein by reference.
Set forth below is a summary of the shortcomings of the application of prior art Hall-Current ion sources for the deposition of DLC and Si-DLC coatings.
1) Inability to produce DLC coatings with hardness greater than 10 GPa and at a deposition rate greater than 10 xc3x85 per second;
2) Inability to operate at relatively high power and discharge current levels for extended periods of time in order to facilitate high deposition rates and beam current fluxes;
3) Operation that is sensitive to the nonconductive hydrocarbon coatings that form within the anode discharge region of the ion source;
4) Ion beam properties that are unreliable, inconsistent in time and spatially asymmetric with respect to the geometry of the ion source apparatus;
5) Susceptible to damaging due to plasma arcs, shorts and other undesirable electrical transient events which inhibit production;
6) Inability to easily ignite and apply to periodic on-off operation;
7) Difficult to scale to applications involving large surface areas or widths.
The method of the present invention comprises depositing a DLC or a Si-DLC coating onto the surface of a substrate using a Hall-Current ion source. This Hall-Current ion source and its embodiments are described in Mahoney, et al., co-pending patent application U.S. Ser. No. 08/901,036 filed on Jul. 25, 1997, the description of which is incorporated herein by reference. Particularly, the ion source incorporates a non-radiative or fluid-cooled anode that provides a conductive surface area or areas where electron contact current can be sustained continuously and substantially uniformly about the anode when depositing DLC and Si-DLC coatings.
In the initial steps of the method, the substrate is mounted in a deposition vacuum chamber containing the Hall-Current ion source and the air is evacuated from the chamber. An inert gas is supplied to at least one self-sustaining cathode electron source and the electron source is suitably excited by power supply means, i.e. one or more power supplies, to provide a supply of electrons to an anode of the Hall-Current ion source. The anode is electrically insulated from the vacuum chamber in such a manner to prohibit the formation of a plasma migrating into the interior of the Hall-Current ion source behind the anode. Plasma maintenance gases are introduced through a gap of the anode and into an anode discharge region within the vacuum chamber. A voltage is then applied to cause a discharge current to flow between the anode and the electron source. Additionally, a voltage is applied to an electromagnet power supply for electromagnetic means, i.e. a least one electromagnet, of the Hall-Current ion source, causing current to flow through the electromagnet. This results in a magnetic field being formed across the anode discharge region and in electrons ionizing the plasma maintenance gases and forming a plasma beam of gas ions throughout the anode discharge region. A layer of DLC or Si-DLC is plasma ion beam deposited from carbon-containing precursor gases or a mixture of silicon-containing and carbon-containing precursor gases, respectively. During the deposition, the anode is thermally cooled by cooling means other than radiative thermal emission, i.e. by the use of convection coolers or conductive coolers. Preferably, the anode is a fluid-cooled anode. The vacuum chamber pressure is increased to atmospheric pressure and a DLC-coated or Si-DLC-coated substrate product is recovered.
The recovered DLC and Si-DLC coatings have hardness values greater than 10 GPa and are deposited at rates substantially greater than 5 xc3x85/sec, i.e., in the range of about 10 to 120 xc3x85/sec., using the unique Hall-Current ion source. This gridless ion source is a closed path or non-closed path Hall-Current ion source that embodies features which overcome the problems encountered with ion sources of the prior art when applied to the production of DLC coatings.
The precursor gases are introduced either through the gap in the anode, along with the plasma maintenance gases, or directly into the plasma beam and separately from the introduction of the plasma maintenance gases through the gap of the anode. The dimensions of the gap within the anode are at least greater than the characteristic Debye length of a local plasma formed near the gap in the anode. The shape of the gap is configured so as to substantially restrict line-of-sight deposition of coating onto the anode within the gap such that the anode discharge current is substantially maintained at the anode within the gap near a localized region of the plasma maintenance gases passing into the anode discharge region.
This unique Hall-Current ion source comprises:
(a) an insulatively sealed anode to prevent plasma from forming behind the anode,
(b) a non-radiative cooling means for cooling the anode;
(c) a self-sustaining cathode, i.e. a cathode having an independent power supply;
(d) an electromagnet that operates at least partially on either the discharge current from the anode to the self-sustaining cathode or on current from an independent, direct current (DC), periodically reversing or alternating current (AC) source; and
(e) at least one gap within the anode to introduce the plasma maintenance gases through an injector or other injection means.
In its application to DLC coating deposition, the Hall-Current ion source apparatus may be configured with either a closed or non-closed Hall-Current drift path region. The non-closed Hall-Current ion source uses a periodically reversing or alternating magnetic field in order to form a plasma beam whose spatial time-averaged output is symmetric with respect to the geometry and scale of the ion source. This non-closed Hal-Current ion source is particularly useful in depositing DLC and Si-DLC coatings onto wide surfaces and substrates.
It has been found that when the DLC or Si-DLC process produces highly insulating deposits on the anode region of the Hall-Current ion source, operating the electromagnet partially on either the discharge current from the anode to the self-sustaining cathode, or on the current from an independent, periodically reversing or alternating current source is preferred over a direct current source. However, it has been found that for certain process conditions which result in weakly insulating or conducting deposits on the anode region, e.g. using acetylene precursor gas, it is possible to operate the Hall-Current source in a stable manner using a direct current source for the electromagnet.
By introducing the precursor gases (with or without nonreactive maintenance gases) completely or in part into the anode discharge region of this Hall-Current ion source, it is possible to deposit DLC coatings at rates in the range of about 10 to about 200 xc3x85 per second, with hardness in the range of about 10 GPa to about 40 GPa and compressive stress in the range of about 0.5 GPa to 8 GPa, and Si-DLC coatings at rates in the range of about 5 to about 200 xc3x85 per second, with hardness in the range of about 10 GPa to about 40 GPa, compressive stress in the range of about 0.5 GPa to about 6 GPa, and silicon concentration in the range of about 2 to about 48 atomic percent. Also with the operation of the unique Hall-Current ion source at high beam current densities, it is possible to deposit unexpectedly hard DLC coatings at surprisingly low time-averaged ion energies, i.e. less than 40 eV, using various hydrocarbon feed gases.